The present invention relates to apparatus for indicating the intended values of signals in digitally based communications channels and, more particularly, to apparatus for determining those intended values, comparing them with the observed values, and, if the intended and observed values differ, acting to reduce the discrepancy.
Communication channels can be entirely analog extending from the information source to the transmitter, and then along the channel link to the receiver, and finally to the user of the transmitted information. Communication channels can also be digitally based, either totally or partially. A typical situation is to have a digital information source but where the information is sent in analog form over an analog channel link to a receiver which reconverts the information to a digital format.
In systems using digital communications channels or communications channels having digital portions along the channel, i.e. digitally based communication channels hereinafter, the expected level of signals arriving at processing apparatus in or following the channel is often important information. For example, in apparatus to monitor channel degradation, the signal levels which are actually observed may be compared with the expected levels. Such a comparison may be used to detect channel degradation before it becomes great enough to introduce errors in the interpretation of the signals.